Efficient Message Transmission and Loop Avoidance in an RPL Network

ABSTRACT

In some implementations, formation of loops between nodes in an RPL network may be avoided. A node that receives a neighbor-discovery message may determine, based on a parent-child status, whether the neighbor-discovery message was received from a parent of the node. If the neighbor-discovery message was received from the parent, the node may transmit an error code to the parent. In addition, the node may determine, based on a comparison of reliability rankings, whether the neighbor-discovery message was received from a node with relatively high reliability, such as a potential grandparent node. If the neighbor-discovery message was received from a high-reliability node, the node may transmit an error code to the high-reliability node. Furthermore, the node may reduce network traffic by withholding a neighbor-detection message from a child node. The node may transmit a response to neighbor-detection messages received from the child node.

TECHNICAL FIELD

This disclosure relates generally to the field of neighbor-discoveryprotocols in computer networks, and more specifically relates toestablishing neighbor relationships between nodes in a computer network.

BACKGROUND

A computer network may include one or more computing devices, or nodes,that communicate with other nodes in the computer network. In somecases, the computer network may implement a routing protocol forlow-power and lossy networks (“RPL network”). In some cases, the RPLnetwork may include one or more devices operating on a limited powersupply (such as a battery), one or more devices operating on a stablepower supply (such as an electrical connection to grid power), ordevices using a combination of limited or stable power supplies. An RPLnetwork may include multiple nodes that transmit and receive messages,such as a message including a packet. A node included in an RPL networkmay transmit and receive messages with neighboring nodes in the network,such as nodes that have a direct communication path with the particularnode. For example, the node may transmit or receive messages to discoverwhich nodes are neighbors with the particular node. In addition, thenode may transmit or receive messages to test a reliability ofcommunication paths with neighboring nodes.

In some cases, the node may transmit routed messages between additionalnodes, such as by relaying a message that is intended to be received byan additional node in the RPL network. The routed messages may betransmitted based on parent-child relationships between nodes, such thatparent nodes route messages to (or from) their child nodes and childnodes route messages to (or from) their parent node. The RPL network mayefficiently transmit messages through the network based on efficientroutes. An efficient route may include more reliable nodes establishingparent relationships, and less reliable nodes establishing non-parentneighbor relationships (such as a child relationship). In addition, anefficient route may avoid loops, such as a node establishing a childrelationship with its own child node (or grandchild node). In somecases, an inefficient route may cause additional messages, such asrepeat messages related to lost packets or messages that are routedthrough a high number of nodes.

Transmitting or receiving messages may cause a node to consume power,such as power related to generating a message for transmission, routinga message, interpreting a received message, or using a communicationsantenna. In an RPL network, transmitting and receiving messages maycause nodes to consume a limited power supply (such as by draining abattery), and may reduce the lifespan of a node. A node that hasconsumed its power supply may be unable to transmit additional messages,and may reduce the capacity of the RPL network to carry messages. Toimprove the capacity of an RPL network, it is desirable to reduce anumber of messages transmitted by nodes in the RPL network. In addition,it is desirable to establish efficient routes in the RPL network,including routes that avoid loops.

SUMMARY

According to certain implementations, a first node in an RPL network mayestablish a first neighbor relationship with a second node and a secondneighbor relationship with a third node. The first neighbor relationshipmay indicate a parent status of the first node and a child status of thesecond node. The second neighbor relationship may indicate a childstatus of the first node and a parent status of the third node. Thefirst node may transmit a neighbor-detection message to the third nodebased on the parent status of the third node, and withhold theneighbor-detection message from being transmitted to the second nodebased on the child status of the second node.

In some implementations, a first node in an RPL network may have aneighbor relationship with a second node in the RPL network. Theneighbor relationship may indicate a parent status of the second nodeand a child status of the first node. The first node may receive fromthe second node a neighbor-discovery message indicating a requestedmodification to the neighbor relationship. Based on the neighborrelationship, the first node may determine that the second node has theparent status. Responsive to determining the parent status of the secondnode, the first node may transmit to the second node a responseincluding an error code indicating that the requested modification isunavailable.

In some implementations, a first node in an RPL network may transmit afirst neighbor-discovery message to multiple additional nodes. The firstnode may receive responses indicating respective transmission metricsand a respective ranking of each responding node. Based on thetransmission metrics, the first node may determine a first rankingindicating a reliability of a communication path of the first node. Thefirst node may receive, from a second node in the RPL network, a secondneighbor-discovery message indicating a second ranking of the secondnode. Based on the first and second rankings, the first node maydetermine that the second node has a higher reliability than the firstnode. Responsive to determining that the second node has the higherreliability, the first node may transmit to the second node a responseincluding an error code.

These illustrative implementations are mentioned not to limit or definethe disclosure, but to provide examples to aid understanding thereof.Additional implementations are discussed in the Detailed Description,and further description is provided there.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, implementations, and advantages of the present disclosure arebetter understood when the following Detailed Description is read withreference to the accompanying drawings, where:

FIG. 1, including FIG. 1a and FIG. 1b , is a block diagram depicting anexample of an RPL network that includes multiple nodes, according tocertain implementations;

FIG. 2 is a diagram depicting an example data flow to reduce a quantityof messages transmitted on the RPL network, according to certainimplementations;

FIG. 3 is a flow chart depicting an example of a process fortransmitting a reduced quantity of neighbor-detection messages in an RPLnetwork, according to certain implementations;

FIG. 4 is a flow chart depicting an example of a process for modifying aneighbor relationship between nodes in an RPL network, according tocertain implementations;

FIG. 5 is a diagram depicting an example data flow to avoid formation ofloops in an RPL network based on a determination of a parent-childstatus, according to certain implementations;

FIG. 6 is a flow chart depicting an example of a process for avoidingformation of loops in an RPL network based on a determination of aparent-child status, according to certain implementations;

FIG. 7 is a diagram depicting an example data flow to avoid formation ofloops in an RPL network based on a comparison of node rankings,according to certain implementations;

FIG. 8 is a flow chart depicting an example of a process for avoidingformation of loops in an RPL network based on a comparison of noderankings, according to certain implementations; and

FIG. 9 is a block diagram depicting an example of a computing deviceconfigured to operate as a node in an RPL network, according to certainimplementations.

DETAILED DESCRIPTION

As discussed above, power consumption of one or more nodes in an RPLnetwork may be improved by reducing a number of messages transmittedbetween the nodes. Certain implementations described herein providetechniques to reduce a quantity of messages that are related to aneighbor discovery protocol implemented within an RPL network, such asby reducing neighbor-discovery messages or neighbor-detection messages.For example, a first node in an RPL network may transmit aneighbor-discovery message to multiple additional nodes in the RPLnetwork, such as to discover which nodes have or can establish a directcommunication path (e.g., are neighbors) with the first node. Based onone or more responses received from the additional nodes, the first nodemay establish neighbor relationships with respect to the additionalnodes, such as neighbor relationships indicating a parent or childstatus. A second node may be indicated as a child of the first node, anda third node may be indicated as a parent of the first node.

In some implementations, the first node may transmit one or moreneighbor-detection messages. For example, neighbor-detection messagesmay be transmitted to test communication paths between neighboringnodes. The first node may transmit a neighbor-detection message to aparent node, such as to detect a reliability of the communication pathwith the parent node. In some cases, the first node may withhold theneighbor-detection message from being transmitted to the child node. Inaddition, the first node may respond to a neighbor-detection messagethat is received from the child node. In the RPL network,neighbor-detection messages may be transmitted by child nodes, and notby parent nodes. In addition, parent nodes may respond to theneighbor-detection messages from the child nodes. As the parent nodesmay not send neighbor-detection messages to the child nodes, the childnodes do not transmit responses. Therefore, the overall number ofneighbor-detection messages, and responses to neighbor-detectionmessages, that are transmitted on the RPL network may be reduced.

In some implementations, a particular node on the RPL network may beseeking a parent node. For example, the particular node may be joiningthe RPL network as a new node, or a communication path between theparticular node and a previous parent node may become unreliable. Theparticular node may transmit a neighbor-discovery message to itsneighbor nodes to establish a new parent node. In some cases, theneighbor nodes may transmit a response to the neighbor-discovery messagebased on a relationship with the particular node. For example, if theparticular node transmits a neighbor-discovery message to its childnode, the child node may determine that the particular node is its ownparent node. The child node may send a response with an error code,based on the determination that the particular node is its parent node.Responsive to receiving the error code, the particular node mayestablish a parent-child relationship with an additional node in the RPLnetwork, such that the additional node is a new parent of the particularnode. Therefore, the RPL network may avoid loops in routes betweennodes.

As used herein, the term “RPL network” may refer to a computer networkthat implements a routing protocol for low-power and lossy computernetworks. An RPL network may, but need not, be a low-power and lossycomputer network. In some cases, an RPL network may include one or morenodes that typically operate with constraints on one or more ofprocessing power, memory, or energy, such as computing devices thatoperate on battery power. In addition, an RPL network may include one ormore nodes characterized by one or more of high loss rates of datamessages (e.g., lost packets), low data rates, or instability ofcommunication paths between nodes on the computer network.

In an RPL network, the routing protocol may establish one or more routesbetween nodes that are included in the RPL network. In addition, the RPLnetwork may establish routes based on neighbor-discovery processesperformed by one or more of the nodes on the RPL network. In some cases,an RPL network may implement neighbor-discovery according to one or morestandards, such as a standard RFC 4861 for “Neighbor Discovery for IPversion 6 (IPv6)” or a standard RFC 6775 for “Neighbor DiscoveryOptimization for IPv6 over Low-Power Wireless Personal Area Networks(6LoWPANs).” In some cases, an RPL network may implement a routingprotocol according to one or more standards, such as a standard RFC 6550for “RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks.”

As used herein, the term “node” may refer to a computing device that isconfigured to receive or transmit messages in a computer network. A nodemay include, without limitation, a router, a gateway, an endpoint, acollector, or any other suitable type of computing device. In somecases, a node may be a virtual node, such as a node that is implementedas a software-based computing device (e.g., a cloud computing network, avirtual server).

As used herein, the term “neighbor discovery” may refer to one or moreoperations performed by a node to determine one or more neighboringnodes in an RPL network. Neighbor discovery may include transmission ofone or more neighbor-discovery messages, such as a neighbor solicitationor a neighbor advertisement. In addition, neighbor discovery may includetransmission of a particular neighbor-discovery message to multiplereceiving nodes, such as a multicast message identified as beingdirected to any node capable of receiving the multicast message. Nodesmay be neighbors with respect to each other if there is a communicationpath between the nodes, such that each node may transmit a message tothe other without routing the message through an additional node. Insome cases, neighbor discovery may be performed by a node that isjoining (or re-joining) an RPL network. In addition, neighbor discoverymay be performed by a node that is included in an RPL network. Forexample, a node that already has one or more neighbors may performneighbor discovery to determine if any additional nodes are available asadditional neighbors.

As used herein, the term “neighbor detection” may refer to one or moreoperations performed by a node included in an RPL network to determineif a neighboring node is reliable. In some cases, neighbor detectionincludes a process for neighbor unreachability detection (“NUD”), suchas a NUD process described by the standard RFC 4861 for “NeighborDiscovery for IP version 6 (IPv6).” Neighbor detection may includetransmission of one or more neighbor-detection messages, such as aneighbor solicitation or a neighbor advertisement. In addition, neighbordetection may include transmission of a particular neighbor-detectionmessage to a particular receiving node, such as a unicast messageidentified as being directed to the particular node. A node may performneighbor detection during other communications with a neighbor, such asby transmitting a neighbor-detection message along with additionalmessages for the neighbor. In addition, a node may perform neighbordetection responsive to determining that the neighbor may be unreliable,such as a neighbor that does not respond to messages.

As used herein, the term “message” and “data message” may refer to agroup of one or more data packets that is configured to be transmittedbetween nodes, such as nodes in an RPL network. A message may includeinformation, such as information about a node that transmits the packet(e.g., performance information for the transmitting node), informationfrom an additional computing system (e.g., a computing system externalto the RPL network), or any other suitable information.

As used herein, the term “communication path” may refer to a linkbetween nodes such that the nodes are capable of transmitting andreceiving messages via the communication path. A communication path maybe established via any suitable communication technique between nodes,such as a wired technique (e.g., network cable) or a wireless technique(e.g., Wi-Fi, Bluetooth, cellular modem), or a combination of wired andwireless techniques.

As used herein, the terms “route” and “routing” may refer to a messagethat is transmitted from an originating node to a recipient node via oneor more additional nodes. For example, a routing node may receive amessage that is intended for a recipient node other than the routingnode. The routing node may transmit the message to the recipient node orto an additional routing node. In some cases, a route may be determinedbased on a routing protocol. In addition, a route may be modified basedon information about nodes included in the route. For example, if arouting node determines that it has lost a connection to its parentnode, the routing node may use neighbor discovery operations todetermine a new parent. In addition, any routes to which the routingnode belongs may be modified based on the new parent of the routingnode.

Referring now to the drawings, FIGS. 1a and 1b (collectively referred toherein as FIG. 1) is a diagram of an example RPL network 100 thatincludes multiple nodes. The nodes in the RPL network 100 may include aroot node 110 and one or more additional nodes, such as the nodes 111,112, 113, 114, 115, 116, 117, 118, and 119. In some cases, each of thenodes 110 through 119 may be configured to transmit and receivemessages. In addition, the root node 110 may be configured tocommunicate with one or more additional computer networks, such as anadditional RPL network or a non-RPL network. In addition, each of thenodes 110 through 119 may have a communication path with one or moreadditional nodes in the RPL network 100. For example, the nodes 110through 119 may be capable of communicating (e.g., transmitting and/orreceiving messages) via wireless communication techniques. For example,the nodes 111 and 112 may be capable of communicating with each other.

Each of the nodes 110 through 119 may perform neighbor discoveryoperations in order to determine one or more neighboring nodes. Theneighbor discovery operations may include transmitting aneighbor-discovery message, such as a neighbor solicitation. In somecases, the neighbor-discovery message may be a multicast message that isdirected to any node that is capable of receiving the neighbor-discoverymessage via a communication path with the node 111. In addition, theneighbor-discovery message may be indicated as not available forretransmission. For example, if the node 112 receives theneighbor-discovery message from the node 111, the node 112 may determinethat the neighbor-discovery message is not to be transmitted to anyadditional nodes. In addition, the neighbor discovery operations mayinclude transmitting a response to the multicast neighbor-discoverymessage, such as a neighbor advertisement. For example, the node 111 maytransmit a neighbor-discovery message via its available communicationpath. The neighbor-discovery message may be received by the nodes 112,113, 114, and 115. Responsive to receiving the neighbor-discoverymessage, the nodes 112, 113, 114, and 115 may each transmit a respectiveresponse to the node 111. In some cases, a response may includeinformation about the responding node, such as information describingthe responding node's Internet protocol address (“IP address”), mediaaccess control address (“MAC address”), ranking, or other suitableinformation about the responding node. In addition, each of the nodes110 through 119 may store information about the additional nodes(including non-neighboring nodes). For example, each node in the RPLnetwork 100 may store a respective IP address and MAC address ofadditional nodes in the RPL network 100. The node information may bestored in any suitable format, such as entries in a lookup table in amemory component of each respective node.

In some implementations, based on the neighbor discovery operations,each of the nodes 110 through 119 may establish one or more neighborrelationships with the additional nodes. FIG. 1a illustrates an exampleset of neighbor relationships that may be established within the RPLnetwork 100. For illustrative purposes, the example neighborrelationships are illustrated via a dashed line between neighboringnodes. For example, the root node 110 may establish respective neighborrelationships with nodes 113 and 114. In addition, node 111 has neighbornodes 112, 113, 114, and 115; node 112 has neighbor nodes 111 and 115;node 113 has neighbor nodes root node 110 and node 111; node 114 hasneighbor nodes root node 110 and nodes 111, 116, and 118; node 115 hasneighbor nodes 111, 112, and 118; node 116 has neighbor nodes 114 and117; node 117 has neighbor nodes 116 and 118; node 118 has neighbornodes 114, 115, 117, and 119; and node 119 has neighbor node 118. Eachnode may store information about its neighbors, such as in the lookuptable.

In some implementations, based on the neighbor discovery operations,each of the nodes 110 through 119 may establish neighbor relationshipsthat indicate a parent status or a child status with respect toadditional nodes in the RPL network 100. In some cases, the parent-childrelationships may be established according to a routing protocol, suchas a routing protocol adhering to one or more industry standards. Inaddition, the parent-child relationships may be established such thatthe nodes 110 through 119 are related in a directed acyclic graph(“DAG”) format. Based on the DAG format, the RPL network 100 has asingle root node (such as the root node 110), each non-root node has asingle parent node, and each non-root node may or may not have one ormore child nodes. Based on the DAG format, the RPL network is directed,wherein network traffic (e.g., messages) is directed between parentnodes and child nodes. Based on the DAG format, the RPL network 100 isacyclic, wherein loops are not formed (e.g., a given node is not bothparent and child of another given node).

FIG. 1b illustrates an example set of parent-child relationships thatmay be established within the RPL network 100. For illustrativepurposes, the example parent-child relationships are illustrated via asolid line between parent and child nodes, wherein an arrow points fromthe child to the parent. The root node 110 may establish one or morechild relationships with one or more additional nodes within the RPLnetwork 100. For example, the root node 110 establishes relationshipswith nodes 113 and 114, wherein the relationships indicate a parentstatus of the node 110 and a child status of the nodes 113 and 114. Inaddition, the root node 110 may establish additional relationships withadditional nodes external to the RPL network 100 (e.g., relationshipswith additional nodes in an additional computer networks).

In some cases, each of the non-root nodes 111 through 119 may establisha parent relationship with one additional node. Furthermore, thenon-root nodes 111 through 119 may establish one or more childrelationships with one or more of the additional nodes. In the RPLnetwork 100, a first node may have a child status with respect to asecond node and a parent status with respect to a third node. Forexample, the node 111 establishes relationships with respect to thenodes 112 and 113. The relationship with node 112 indicates a parentstatus of node 111 and a child status of node 112. The relationship withnode 113 indicates a child status of node 111 and a parent status ofnode 113. Table 1 summarizes the example parent-child relationshipsillustrated in FIG. 1b . In some cases, a peer status is indicated for anode that has a neighbor relationship, and is not a parent or a child.For example, nodes 115 and 118 may have a neighbor relationship (such asindicated in FIG. 1a ) where neither node 115 or node 118 is the parentor child of the other (such as indicated by Table 1). In addition, thenodes 115 and 118 may establish a neighbor relationship indicating apeer status with respect to each other.

TABLE 1 Node Parent of node Child(ren) of node Root Node 110 (none) 113,114 Node 111 113 112 Node 112 111 115 Node 113 Root node 110 111 Node114 Root node 110 116, 118 Node 115 112 (none) Node 116 114 117 Node 117116 (none) Node 118 114 119 Node 119 118 (none)

In some implementations, based on the neighbor discovery operations,each of the nodes 110 through 119 may determine a respective rankingthat indicates the respective node's relative reliability within the RPLnetwork 100. For each respective node, the ranking may indicate thenode's ability to transmit messages through the RPL network 100 relativeto additional nodes in the RPL network 100. In some cases, eachparticular node may determine its own ranking based on performanceinformation about the RPL network 100, including performance informationdescribing transmission metrics of one or more of the nodes (e.g., timeto transmit a message between nodes), a quantity of routing nodes (e.g.,“hops”) between the particular node and the root node 110, a quantity oflost messages between the particular node and additional nodes, or anyother suitable performance information. The ranking may be calculated asa numeric value. In some cases, the ranking is calculated to be asmaller value based on higher reliability and a larger value based onlower reliability. For example, the root node 110 may have a ranking of0 based on the ability of the root node 110 to transmit messages toitself with very high reliability. The nodes 113 and 114 may haverelatively small ranking numbers of 10 and 15, respectively, based ontheir ability to transmit messages to the root node 110 with relativelyhigh reliability. The nodes 115 and 119 may have relatively largeranking numbers of 38 and 45, respectively, based on their ability totransmit messages to the root node 110 with relatively poor reliability.Table 2 summarizes example rankings of the nodes 110 through 119illustrated in FIG. 1. Although the example rankings in Table 2 indicatesmaller values based on a higher reliability, other implementations maybe used, such as rankings calculated to be smaller values based on lowerreliability and larger values based on higher reliability.

TABLE 2 Node Ranking Root Node 110 0 Node 113 10 Node 114 15 Node 111 20Node 116 23 Node 118 27 Node 117 30 Node 112 34 Node 115 38 Node 119 45

In some implementations, each of the nodes 110 through 119 may performneighbor detection operations in order to detect a reliability of aneighboring node. The neighbor detection operations may includetransmitting a neighbor-detection message, such as a neighborsolicitation. In some cases, the neighbor-detection message may be aunicast message that is directed to a particular neighboring node. Inaddition, the neighbor detection operations may include transmitting aresponse to the unicast neighbor-detection message, such as a neighboradvertisement. For example, the node 111 may transmit a detectionmessage to each of the neighboring nodes 112, 113, 114, and 115.Responsive to receiving the detection message, the nodes 112, 113, 114,and 115 may each transmit a respective response to the node 111. In somecases, a response may include information about the responding node,such as information describing the responding node's ranking, or othersuitable information about the responding node.

In some cases, a neighbor relationship in the RPL network 100 may bemodified. A particular node may determine (such as based on neighbordetection operations) that a neighboring node is non-responsive. Forexample, a communication path between the particular node in theneighboring node may be interrupted. In addition, the particular nodemay determine that the neighboring node has a ranking that is lessreliable than the ranking of the particular node. For example, theneighboring node may experience a failure or other change in its ownoperating conditions that has changed the ranking of the neighboringnode to indicate a relatively low reliability. Based on determining thatthe neighboring node is non-responsive or that the ranking of theneighboring node has changed, the particular node may update itsneighbor relationship with respect to the non-responsive node. Forexample, the particular node may revoke a parent status of thenonresponsive node and attempt to establish a new parent relationshipwith an additional node in the RPL network 100.

In an example implementation, the node 111 may modify a neighborrelationship with respect to the node 113. For instance, and not by wayof limitation, the node 111 may fail to receive, from the node 113, aresponse to a neighbor-detection message sent by the node 111 (e.g., thenode 113 is unresponsive, a connection path is broken). In addition, thenode 113 may indicate (e.g., via a neighbor-discovery message) that itis seeking a new parent. The node 111 may modify the neighborrelationship with the node 113 to revoke the parent status of the node113.

In addition, the node 111 may perform one or more neighbor discoveryoperations to determine a new parent node. In some cases, the node 111may transmit a neighbor-discovery message, such as a multicastneighbor-discovery message. The node 111 may receive responses fromadditional nodes in the RPL network 100, such as responses from theneighboring nodes 112, 114, and 115. In addition, the node 111 maymodify one or more of the neighbor relationships with the nodes 112,114, and 115 based on information included in the responses to theneighbor-discovery message.

In some cases, a node may provide a response to a neighbor-discoverymessage based on a parent-child relationship with the node thattransmitted the neighbor-discovery message. For example, the node 112may receive the neighbor-discovery message from the node 111. Responsiveto receiving the neighbor-discovery message, the node 112 may determinethat it has a child status with respect to the node 111. Based ondetermining the child status, the node 112 may transmit to the node 111a response that includes an error code. The error code may indicate, forexample, that the node 112 is not available as a parent node for thenode 111. Based on receiving the response with the error code, the node111 may establish (or retain) a non-parent neighbor relationship withthe node 112. In some cases, determining whether to modify arelationship based on an error code avoids forming a loop in the RPLnetwork 100 by avoiding establishing a parent relationship with a childnode.

In addition, a node may provide a response to a neighbor-discoverymessage based on a ranking of the node. For example, the node 115 mayreceive the neighbor-discovery message from the node 111. Responsive toreceiving the neighbor-discovery message, the node 115 may transmit tothe node 111 a response that includes a ranking of the node 115. Basedon receiving the response with the ranking, the node 111 may compare theranking of the node 115 to a ranking of the node 111. Based on adetermination that the node 115 has a less reliable ranking compared tothe node 111, the node 111 may establish (or retain) a non-parentneighbor relationship with the node 115. In some cases, determiningwhether to modify a relationship based on a ranking avoids forming aloop in the RPL network 100 by avoiding establishing a parentrelationship with a node that has relatively low reliability.

The node 114 may provide an additional response to theneighbor-discovery message. The response from the node 114 may omit anerror code, such as an error code indicating that the node 114 is notavailable as a parent. In addition, the response from the node 114 mayinclude a ranking of the node 114. The node 111 may determine, based ona comparison of the rankings of the node 114 and the node 111, that thenode 114 has a more reliable ranking compared to the node 111. Based onthe response omitting the error code and on the determination that thenode 114 has a more reliable ranking, the node 111 may establish aneighbor relationship with respect to the node 114 indicating a parentstatus of the node 114 and a child status of the node 111.

Reducing a Quantity of Neighbor-Detection Messages

FIG. 2 depicts an example of a data flow for the RPL network 100,according to certain implementations. In some implementations, the dataflow depicted in FIG. 2 may reduce a quantity of messages transmitted onthe RPL network 100, such as by reducing a number of neighbor-detectionmessages, and responses to neighbor-detection messages, that aretransmitted. In some cases, reducing the quantity of messages mayimprove the capacity of the RPL network 100, such as by extending abattery life of one or more nodes on the network, or by allocatingincreased network capacity to carry additional messages that are notneighbor-detection messages.

In FIG. 2, the node 111 may transmit a neighbor-discovery message to oneor more nodes. For example, at step 202 a, the node 111 may transmit theneighbor-discovery message to node 113. At steps 202 b and 202 c, thenode 111 may transmit the neighbor-discovery message to node 112 andnode 114, respectively. In some cases, the neighbor-discovery messagemay be a multicast message, that is directed to multiple recipientnodes. For example, the node 111 may transmit the multicastneighbor-discovery message once in regards to steps 202 a, 202 b, and202 c, and the multicast neighbor-discovery message may be received byeach of the nodes 113, 112, and 114.

At step 204 a, the node 113 transmits a response to the node 111. Inaddition, the node 112 transmits a response at step 204 b, and node 114transmits a response at step 204 c. The responses may each be a unicastmessage that is directed to the node 111. In regards to steps 204 a, 204b, and 204 c, the responses may include information about the respectiveresponding node, such as an IP address, a MAC address, a ranking, orother suitable information about the responding node.

The node 111 may receive the responses transmitted by the nodes 113,112, and 114. Based on the responses, the node 111 may establish one ormore neighbor relationships with respect to the nodes 113, 112, and 114.At step 206, for example, the node 111 may establish a neighborrelationship indicating a parent-child status with respect to the node113. The neighbor relationship established at step 206 may indicate aparent status of the node 113 and a child status of the node 111 (e.g.,node 113 is the parent of child node 111). In some cases, additionalneighbor relationships may be established in the RPL network 100. Forexample, at step 208 the node 112 may establish an additional neighborrelationship indicating a parent-child status with respect to the node111. The additional neighbor relationship established at step 208 mayindicate a parent status of the node 111 and a child status of the node112 (e.g., node 111 is the parent of child node 112). Some nodes in theRPL network 100 may have both a parent status and a child status, suchas the node 111 being a parent of node 112 and a child of node 113.

In the RPL network 100, a node may transmit one or moreneighbor-detection messages based on the status of the node in aneighbor relationship, such as a parent-child relationship. For example,the node 111 may determine that it has child status with respect to node113. At step 210, the node 111 may transmit a neighbor-detection messageto the node 113, based on the node 113 having parent status with respectto the node 111. In addition, at step 212 the node 113 may transmit aresponse to the neighbor-detection message that was transmitted at step210. At step 214, the node 112 may transmit an additionalneighbor-detection message to the node 111, based on the node 111 havingparent status with respect to the node 112. In addition, at step 216 thenode 111 may transmit an additional response to the additionalneighbor-detection message that was transmitted at step 214.

In some implementations, a node in the RPL network 100 may withhold aneighbor-detection message based on the status of the node in a neighborrelationship, such as a parent-child relationship. For example, inregards to step 210, the node 111 may withhold transmission of aneighbor-detection message to the node 112, based on the node 112 havingchild status with respect to the node 111. Withholding a message mayinclude not generating a message, or not transmitting a message that isgenerated. For example, the node 111 may be configured to, based on thechild status of node 112, perform (or avoid performing) operationsrelated to generating or transmitting a neighbor-detection message fornode 112. Node 111 may avoid generating a neighbor-detection message tonode 112, or may transmit a generated neighbor-detection message to afirst node (e.g., node 113) and avoid transmitting the generatedneighbor-detection message to a second node (e.g., node 112).

In regards to step 212, the node 112 may be configured to not generate aresponse, as it did not receive the withheld neighbor-detection message.In addition, the node 113 may withhold transmission of an additionalneighbor-detection message to the node 111, based on the node 111 havingchild status with respect to the node 113, and the node 111 may withholda response. In some cases, withholding neighbor-detection messages basedon a status of a node in a neighbor relationship may reduce a quantityof neighbor-detection messages and a quantity of responses toneighbor-detection messages that are transmitted on the RPL network 100.

In some implementations, a node in the RPL network 100 may modify aneighbor relationship with a neighboring node, such as to change a childnode to a parent node, or a parent node to a child node. The neighborrelationship may be modified based on information indicated by a messagefrom the neighboring node, such as a neighbor-discovery message. Inaddition, the neighbor relationship may be modified based on a status ofthe neighboring node, such as a parent or child status in the neighborrelationship (e.g., the unmodified neighbor relationship). The exampledata flow depicted in FIG. 2 illustrates an example modification of aneighbor relationship in an environment in which neighbor-detectionmessages are reduced based on a parent-child status in a neighborrelationship. However, other implementations are possible, such asmodifying a neighbor relationship in an environment in whichneighbor-detection messages are not reduced.

At step 230 in FIG. 2, the node 113 may lose a connection to its ownparent node, such as a node having a parent status with respect to thenode 113. For instance, the node 113 may lose a connection to root node110, indicated in Table 1 as the example parent of node 113. In somecases, the node 113 may attempt to find a new parent node. For example,at step 232, the node 113 may transmit a neighbor-discovery message toone or more nodes in the RPL network 100, such as to the node 111. Insome cases in regards to step 232, the neighbor-discovery message may bea multicast message, such as a multicast message that is directed tomultiple recipient nodes. In addition, the neighbor-discovery message inregards to step 232 may include an error code, such as an errorindicating that the node 113 does not have a parent.

The node 111 may receive the neighbor-discovery message transmitted atstep 232. In addition, the node 111 may determine that the transmittingnode 113 has a parent status with respect to the node 111. Responsive todetermining that the node 113 has the parent status, the node 111 maymodify the neighbor relationship with the node 113, such as at one ormore of steps 234 or 238. In addition, the node 111 may establish (ormodify) one or more additional neighbor relationships with additionalnodes in the RPL network 100, such as at step 236.

At step 234, for example, the node 111 may revoke the parent status ofthe node 113. Revoking the parent status may include modifying theneighbor relationship with respect to node 113 to indicate that the node113 has no status, or has an additional status that is not parent (e.g.,child, peer).

At step 236, the node 111 may establish or modify an additional neighborrelationship with the node 114. The additional neighbor relationship mayindicate a parent status of the node 114 and a child status of the node111 (e.g., node 114 is the new parent of child node 111). In some cases,the additional neighbor relationship is established or modified based onone or more additional messages, such as a multicast neighbor-discoverymessage that is transmitted by node 111 for the purpose of finding a newparent. In addition, the additional neighbor relationship is establishedor modified based on previously received information, such as theresponse from node 114 received in regards to step 204 c.

At step 238, the node 111 may modify the neighbor relationship with thenode 113 to indicate an additional parent status of the node 111 and achild status of the node 113 (e.g., node 111 is the new parent of childnode 113). Although FIG. 2 depicts performing this modification at step238, other implementations are possible, such as modifying the neighborrelationship at step 234 to indicate the child status of the node 113.

FIG. 3 is a flow chart depicting an example of a process 300 fortransmitting a reduced quantity of neighbor-detection messages in an RPLnetwork. In some implementations, such as described in regards to FIGS.1-2, a computing device configured as a node in an RPL networkimplements operations described in FIG. 3, by executing suitable programcode. For illustrative purposes, the process 300 is described withreference to the examples depicted in FIGS. 1-2. Other implementations,however, are possible.

At block 310, the process 300 involves determining, by a node in an RPLnetwork, a group of neighbor nodes in the RPL network. The node maydetermine the neighbor nodes based on a transmitted neighbor-discoverymessage, such as a neighbor-discovery message transmitted by the node111 in regards to steps 202 a, 202 b, or 202 c. The neighbor-discoverymessage may be transmitted as a multicast message. In addition, theneighbor-discovery message may be indicated as not available forretransmission. In some cases, the multicast neighbor-discovery messagemay be transmitted to additional nodes having direct communication linkswith the transmitting node and not retransmitted to any further nodesnot having direct communication links with the transmitting node.

In some implementations, the node may receive responses from one or moreadditional nodes that receive the neighbor-discovery message, such asneighbor nodes having direct communication links with the node. Inaddition, the node may establish neighbor relationships based on, atleast, information included in the responses. In addition, the node maymanage the neighbor relationships with each of the neighboring nodes,such as via one or more neighbor-detection messages. Theneighbor-detection messages may be transmitted as unicast messages,including a generated neighbor-detection message that is transmittedmultiple times (e.g., to each neighbor node), or multiple generatedneighbor-detection messages that are respectively transmitted to arespective one of the neighboring nodes. The neighboring nodes may beconfigured to respond to the neighbor-detection message. In some cases,the node may manage the neighbor relationships based on such responses.

At block 320, the process 300 involves establishing, by the node, afirst neighbor relationship with respect to a first neighbor node in thegroup of neighbor nodes. The first neighbor relationship may indicateone or more of a parent status of the node or a child status of thefirst neighbor node. At block 330, the process 300 involvesestablishing, by the node, a second neighbor relationship with respectto a second neighbor node in the group of neighbor nodes. The secondneighbor relationship may indicate one or more of a child status of thenode or a parent status of the second neighbor node. For example, thenode 111 may establish neighbor relationships with each of nodes 112 and113. The first neighbor relationship with respect to node 112 mayindicate a parent status of node 111 and a child status of node 112. Thesecond neighbor relationship with respect to node 113 may indicate aparent status of node 113 and a child status of node 111. In someimplementations, operations described with regards to blocks 320 and 330may be performed in various orders. For example, the node may establisha neighbor relationship with a parent node prior to establishing anotherneighbor relationship with one or more child nodes (e.g., the node mayestablish a parent, as generally described in regards to block 330,before establishing any children, as generally described in regards toblock 320).

At block 340, the process 300 involves transmitting, by the node, aneighbor-detection message to a subset of the group of neighbor nodes.The subset may exclude the first neighbor node based on the firstneighbor node having the child status with respect to the node. Inaddition, the subset may include the second neighbor node based on thesecond neighbor node having the parent status with respect to the node.In some cases, additional neighbor nodes in the group may be included inthe subset based on their having a non-child status (e.g., peer status)with respect to the node.

At block 350, the process 300 involves responding, by the node, to anadditional neighbor-detection message received from the first neighbornode. For example, responsive to receiving from the node 112 aneighbor-detection message, node 111 may transmit a response to the node112. In some cases, operations related to block 350 may be optional,such as if the first neighbor node does not send an additionalneighbor-detection message to the node.

In some implementations, a node in an RPL network may modify a neighborrelationship with a neighboring node, such as to change a child node toa parent node, or a parent node to a child node. FIG. 4 is a flow chartdepicting an example of a process 400 for modifying a neighborrelationship between nodes in an RPL network. In some implementations,such as described in regards to FIGS. 1-3, a computing device configuredas a node in the RPL network implements operations described in FIG. 4,by executing suitable program code. For illustrative purposes, theprocess 400 is described with reference to the examples depicted inFIGS. 1-3. Other implementations, however, are possible.

At block 410, the process 400 involves receiving a neighbor-discoverymessage in an RPL network. The neighbor-discovery message may bereceived by a node in the RPL network, and transmitted by a firstneighbor node in the RPL network. In addition, the node and the firstneighbor node may have a neighbor relationship with respect to eachother, such as a neighbor relationship indicating a parent-child statusof the node or of the first neighbor node. For example, node 111 mayreceive a neighbor-discovery message from the node 113 in the RPLnetwork 100. The node 111 may also have a neighbor relationship with thenode 113, indicating a parent status of the node 113 and a child statusof the node 111.

In some cases in regards to block 410, the neighbor-discovery messagemay include or otherwise indicate an error code. For example, the node111 may receive, included with the neighbor-discovery message, an errorcode indicating that the node 113 does not have a parent node, or thatthe node 113 is seeking a new parent node, or additional informationrelated to the error code.

At block 420, the process 400 involves determining, by the node, aparent status of the first neighbor node with respect to the node. Theparent status may be determined based on the neighbor relationship withrespect to the first neighbor node. For example, responsive to receivingthe neighbor-discovery message from the node 113, node 111 may determinewhether node 113 has a parent status with respect to the node 111, basedon the neighbor relationship with respect to the node 113.

At block 430, the process 400 involves modifying, by the node, theneighbor relationship with respect to the first neighbor node. Themodified neighbor relationship may revoke the parent status of the firstneighbor node with respect to the node. In addition, the modifiedneighbor relationship may indicate one or more of a parent status of thenode or a child status of the first neighbor node. For example, the node111 may modify the neighbor relationship with node 113 to revoke theparent status of node 113, to indicate an additional parent status ofthe node 111, to indicate a child status of the node 113, or anycombination of such modifications.

At block 440, the process 400 involves establishing, by the node, anadditional neighbor relationship with a second neighbor node of the RPLnetwork. The additional neighbor relationship may indicate one or moreof a parent status of the first neighbor node or a child status of thenode. The node 111, for example, may establish an additional neighborrelationship with respect to node 114, indicating a parent status of thenode 114 and a child status of the node 111.

Preventing Formation of Loops

FIG. 5 depicts an example of a data flow for the RPL network 100,according to certain implementations. In some implementations, the dataflow depicted in FIG. 5 may avoid the formation of loops in routesbetween nodes of the RPL network 100. For example, a node that isseeking a parent may request to become the child node of its own child,such as by sending a neighbor-discovery message. In some cases, stepsdescribed in regards to FIG. 5 may prevent the node from establishing achild relationship with its own child. In addition, steps described inregards to FIG. 5 may cause the node to establish a parent relationshipwith an additional node (e.g., not a child of the node).

In FIG. 5, the node 111 may establish a neighbor relationship withrespect to one or more additional nodes in the RPL network 100. Forexample, at step 502, the node 111 may establish a neighbor relationshipindicating a parent-child status with respect to the node 113. Theneighbor relationship established at step 502 may indicate a parentstatus of the node 113 and a child status of the node 111 (e.g., node113 as the parent of child node 111). The node 111 may have both parentstatus and a child status in the RPL network 100. For example, at step504 the node 112 may establish an additional neighbor relationshipindicating a parent-child status with respect to the node 111. Theadditional neighbor relationship established at step 504 may indicate aparent status of the node 111 and a child status of the node 112 (e.g.,node 111 is the parent of child node 112).

At step 506 in FIG. 5, the node 113 may lose a connection to its ownparent node. For instance, the node 113 may lose a connection to theroot node 110, indicated in Table 1 as the example parent of node 113.In some cases, the node 113 may attempt to find a new parent node. Forexample, at step 508 the node 113 may transmit a neighbor-discoverymessage to one or more nodes in the RPL network 100, such as to the node111. In some cases in regards to step 508, the neighbor-discoverymessage may be a multicast message that is directed to multiplerecipient nodes. The multicast message may be received by recipientnodes that have a communication path with the node 113 (e.g., capable ofreceiving the multicast message directly, without retransmission). Inaddition, the neighbor-discovery message in regards to step 508 mayinclude an error code, such as an error indicating that the node 113does not have a parent.

The node 111 may receive the neighbor-discovery message transmitted atstep 508. The node 111 may determine that the transmitting node 113 hasa parent status with respect to the node 111. Responsive to determiningthat the node 113 has the parent status, node 111 may transmit aresponse to the neighbor-discovery message, such as at step 510. Theresponse may be a unicast message that is directed at the node 113. Theresponse in regards to step 510 may include an error code, such as anerror code indicating that the node 111 is not available as a parentnode for the node 113. In some cases, the error code in regards to step510 may indicate a status of a memory cache, such as a “Neighbor CacheFull” error code that is compliant with the standard RFC 6775 for“Neighbor Discovery Optimization for IPv6 over Low-Power WirelessPersonal Area Networks (6LoWPANs).” In addition, the error code inregards to step 510 may indicate that no route upwards (e.g., a routetowards the root node 110) is available to the requesting node, such asa “No Route” error code that is described in addition to error codesdescribed in the standard RFC 6775. Based on receiving the response withthe error code at step 510, the node 113 may establish (or retain) aneighbor relationship indicating a child status of the node 111. Inaddition, the node 113 may establish (or attempt to establish) a parentneighbor relationship with an additional node in the RPL network 100. Insome cases, establishing or maintaining a child neighbor relationshipwith a child node based on an error code received from the child nodemay prevent a loop from forming in the RPL network 100, such as a loopbetween the node 111 and the node 113.

In some implementations of an RPL network, a node that is seeking aparent node may determine a value of a counter associated with anadditional node that has provided a response with an error code. Forexample, at step 512 the node 113 may determine a value of a counterassociated with the node 111. In some cases, the counter may beinitialized by the node 113 responsive to receiving the response withthe error code in regards to step 510. The initialized counter may havea value based on the response received at step 510. In addition, thenode 113 may have a counter already running for the node 111, and thenode 113 may check the value of the running counter. In some cases, thecounter may be a lifetime registration counter, such as a registrationcounter that is based on a lifetime value indicated in the responsereceived at step 510. For example, the lifetime value may comply withthe standard RFC 4861 for “Neighbor Discovery for IP version 6 (IPv6).”

The counter initialized at step 510 may count (e.g., increment ordecrement) for a period of time, such as a period of time indicated bythe lifetime value. In some cases, the node 113 withholds additionalneighbor-discovery messages from the node 111 during the period of timeassociated with the counter. For example, if the response transmitted bythe node 111 at step 510 indicates that the node 111 has a particularlifetime value indicating a particular period of time (e.g., one day),the node 113 withholds additional neighbor-discovery messages from thenode 111 during the particular period of time. If the node 113 is stillseeking a parent upon completion of the counter, the node 113 maytransmit additional neighbor-discovery messages to the node 111 afterthe particular period of time (e.g., after one day).

In some implementations, the node that is seeking a parent node may beunable to find a new parent node that is not a child of the node seekinga parent. The node may transmit a deregistration message. Thederegistration message may be transmitted to an additional node that haspreviously provided a response with an error code, such as a child ofthe node seeking a parent. In addition, the deregistration message mayindicate a modification of a neighbor relationship, such as a revocationof a child status of the additional node. For example, at step 514 thenode 113 may transmit a deregistration message to the node 111. Thederegistration message may include an error code indicating amodification of the neighbor relationship between the node 113 in thenode 111. In some cases, node 111 may modify the neighbor relationshipwith respect to node 113 to revoke the child status of node 111 and/orthe parent status of node 113. In addition, node 111 may modify theneighbor relationship with respect to node 113 to indicate an additionalstatus (e.g., peer, child) of the node 113.

At step 516, the node 113 may transmit an additional neighbor-discoverymessage to the node 111. The additional neighbor-discovery message mayindicate that the node 113 is seeking a parent node. Based on theadditional neighbor-discovery message, the node 111 may transmit to thenode 113 a response that indicates the node 111 is available as a parent(e.g., a response omitting an error code). In addition, the node 113 mayestablish (or modify) the neighbor relationship with the node 111indicating a parent status of the node 111 and a child status of thenode 113. In some cases, the deregistration message in regards to step514 and the additional neighbor-discovery message in regards to step 516may be transmitted during the period of time indicated by the counterinitialized at step 512. For example, if the node 113 is unable to finda new parent during the period of time associated with the counter, thenode 113 may transmit the deregistration message or the additionalneighbor-discovery message while the counter is running. In some cases,transmitting the deregistration message and the additionalneighbor-discovery message may allow the node 113 to establish a newroute to the root node 110, via node 111, while a counter based on thelifetime value of node 111 is running.

In some implementations of an RPL network, a node that receives an errorcode may establish a neighbor relationship with an additional node(e.g., an additional node that did not provide the error code). At step530, for example, the node 111 may lose a connection to its parent node.Responsive to determining that the connection to the parent node islost, the node 111 may attempt to find a new parent node. For example,at step 532, the node 111 may transmit a neighbor-discovery message toone or more nodes in the RPL network 100, such as to the node 112. Insome cases in regards to step 532, the neighbor-discovery message may bea multicast message that is directed to multiple recipient nodes. Inaddition, the neighbor-discovery message at step 532 may include anerror code, such as an error code indicating that the node 111 does nothave a parent.

At step 534, the node 111 may receive from the node 112 a response thatincludes an error code indicating that the node 112 is not available asa parent node for the node 111. In some cases, the error code at step534 is the same, or similar to, the error code described at step 510,such as a particular error code indicating unavailability as a parent.Based on receiving the response with the error code at step 534, thenode 111 may establish (or retain) a neighbor relationship with the node112, such as a neighbor relationship indicating a child status of thenode 112 and a parent status of the node 111. In some cases, the node111 may initialize a counter based on the response received at step 534,such as a lifetime registration counter indicating a period of timeduring which additional neighbor-discovery messages are withheld fromthe node 112.

Responsive to receiving the response with the error code at step 534,the node 111 may establish (or attempt to establish) a parent neighborrelationship with an additional node in the RPL network 100. Forexample, at step 536, the node 111 may establish an additional neighborrelationship with respect to the node 114. The additional neighborrelationship with the node 114 may indicate a parent status of the node114 and a child status of the node 111. In some cases, the additionalneighbor relationship is established or modified based on one or moreadditional messages, such as a multicast neighbor-discovery message thatis transmitted by node 111 for the purpose of finding a new parent. Inaddition, the additional neighbor relationship is established ormodified based on previously received information, such as a responsefrom node 114 to a previously transmitted neighbor-discovery message. Insome cases, establishing a parent neighbor relationship with anadditional node based on an error code received from a child node mayprevent a loop from forming in the RPL network 100, such as a loopbetween the node 111 and node 112.

FIG. 6 is a flow chart depicting an example of a process 600 foravoiding formation of loops in an RPL network based on a determinationof a parent-child status. In some implementations, such as described inregards to FIGS. 1-5, a computing device configured as a node in an RPLnetwork implements operations described in FIG. 6, by executing suitableprogram code. For illustrative purposes, the process 600 is describedwith reference to the examples depicted in FIGS. 1-5. Otherimplementations, however, are possible.

At block 610, the process 600 involves establishing, by a first node inan RPL network, a first neighbor relationship with respect to a secondnode in the RPL network. The first neighbor relationship may indicate aparent status of the first node or a child status of the second node. Atblock 620, the process 600 involves establishing, by the first node, asecond neighbor relationship with respect to a third node in the RPLnetwork. The second neighbor relationship may indicate a child status ofthe first node or a parent status of the third node. For example, thenode 111 may establish (or retain) neighbor relationships with each ofthe nodes 112 and 113. The first neighbor relationship with respect tonode 112 may indicate a parent status of node 111 and a child status ofnode 112. The second neighbor relationship with respect to node 113 mayindicate a parent status of node 113 and a child status of node 111. Insome implementations, operations described with regards to blocks 610and 620 may be performed in various orders. For example, the node mayestablish a neighbor relationship with a parent node prior toestablishing another neighbor relationship with one or more child nodes(e.g., the node may establish a parent, as generally described inregards to block 620, before establishing any children, as generallydescribed in regards to block 610).

At block 630, the process 600 involves receiving, by the first node, aneighbor-discovery message transmitted from the third node, such as aneighbor-discovery message indicating that the third node is seeking aparent. The neighbor-discovery message may be received as a multicastmessage. In addition, the neighbor-discovery message may indicate arequested modification of the second neighbor relationship. In somecases, fulfilling the requested modification would revise the status ofthe first or third nodes (e.g., the first node's status would be revisedto parent, the third node's status would be revised to child). Forexample, the node 111 may receive from the node 113 a neighbor-discoverymessage indicating that the node 113 is seeking a parent node, such asat step 508.

At block 640, the process 600 involves determining, by the first node,the parent status of the third node. In some cases, the first nodedetermines the parent status based on the second neighbor relationshipwith respect to the third node. For example, responsive to receiving aneighbor-discovery message from node 113, node 111 may determine thatthe node 113 has a parent status with respect to node 111.

At block 650, the process 600 involves transmitting, by the first nodeand to the third node, a response to the neighbor-discovery message. Theresponse may include an error code, such as an error code indicatingthat the requested modification of the second neighbor relationship isnot available. For example, the error code may indicate that the firstnode is not available as a parent for the third node. For example, thenode 111 may transmit to the node 113 a response with an error code,such as at step 510. In some cases, the response transmitted by thefirst node may indicate a lifetime value for the first node, such as alifetime value by which the third node may initialize a counter.

At block 660, the process 600 involves transmitting, by the first node,an additional neighbor-discovery message to the second node. Theadditional neighbor-discovery message may be transmitted as a multicastmessage. The additional neighbor-discovery message may indicate arequested modification of the first neighbor relationship, such thatfulfilling the requested modification would revise the status of thefirst or second nodes (e.g., the first node's status would be revised tochild, the second node's status would be revised to parent). In somecases, the first node may transmit the additional neighbor-discoverymessage based on having lost a connection to a node with which the firstnode has a parent relationship, such as the third node. For example, thenode 111 may transmit a neighbor-discovery message to node 112, such asat step 532, based on having lost its parent connection at step 530. Inaddition, the neighbor-discovery message in regards to step 532 mayindicate a requested modification to the neighbor relationship betweennodes 111 and 112.

At block 670, the process 600 involves receiving, by the first node andfrom the second node, an additional response to the additionalneighbor-discovery message. The additional response may include an errorcode, such as an error code indicating that the requested modificationof the first neighbor relationship is not available. For example, theerror code may indicate that the second node is not available as aparent for the first node. For example, the node 111 may receive fromthe node 112 a response with an error code, such as at step 534. In somecases, the additional response transmitted by the second node mayindicate a lifetime value for the second node. In addition, the firstnode may initialize a counter associated with the second node. During aperiod of time indicated by the counter (e.g., while the counter isincrementing or decrementing), the first node may withhold furtherneighbor-discovery messages from the second node.

At block 680, the process 600 involves establishing, by the first node,a third neighbor relationship with respect to a fourth node in the RPLnetwork. The third neighbor relationship may be established responsiveto receiving the additional response with the error code. In addition,the third neighbor relationship may indicate a parent status of thefourth node or a child status of the first node. For example, the node111 may establish (or modify) a neighbor relationship with the node 114,such as at step 536.

Although the blocks 660, 670, and 680 are described in regards to blocks610 through 650, other implementations are possible. For example, a nodethat transmits a neighbor-discovery message for the purpose of finding anew parent, such as generally described in regards to blocks 660 through680, may, but need not, receive or respond to another neighbor-discoverymessage, such as generally described in regards to blocks 610 through650.

FIG. 7 depicts an example of a data flow for the RPL network 100,according to certain implementations. In some implementations, the dataflow depicted in FIG. 7 may avoid the formation of loops in routesbetween nodes of the RPL network 100, such as by causing a node toestablish a parent relationship with an additional node having a morereliable ranking. For example, a node that is seeking a parent mayrequest to become the child node of a node with a low reliability, suchas a grandchild node. In some cases, steps described in regards to FIG.7 may prevent from the node from establishing a child relationship witha grandchild node or other low-reliability node. In addition, stepsdescribed in regards to FIG. 7 may cause the node to establish a parentrelationship with an additional node having a high reliability (e.g.,not a grandchild node)

In FIG. 7, the node 111 may transmit a neighbor-discovery message to oneor more nodes in the RPL network 100. For example, at step 702 a, thenode 111 may transmit the neighbor-discovery message to node 113. Atsteps 702 b and 702 c, the node 111 may transmit the neighbor-discoverymessage to node 112 and node 114, respectively. In some cases, theneighbor-discovery message may be a multicast message that is directedto multiple recipients. For example, the node 111 may transmit themulticast neighbor-discovery message once in regards to steps 702 a, 702b, and 702 c, and the multicast neighbor-discovery message may bereceived by each of the nodes 113, 112, and 114.

At step 704 a, the node 113 transmits a response to the node 111. Inaddition, the node 112 transmits a response at step 704 b, and node 114transmits a response at step 704 c. The responses may each be a unicastmessage that is directed to the node 111. In regards to the steps 704 a,704 b, and 704 c, the responses may include information about therespective responding node, such as a ranking, an IP address, a MACaddress, or other suitable information about the responding node. Insome cases, the node 111 receives transmission metrics associated withone or more of the responding nodes 113, 112, or 114. The transmissionmetrics may describe a reliability with which the responding nodetransmits or routes messages to the root node 110. For example, thetransmission metrics may describe a time to transmit a message betweenthe responding node and an additional node, a quantity of routing nodes(e.g., “hops”) between the responding node and the root node 110, aquantity of lost messages between the responding node and additionalnodes, or any other suitable information describing a reliability of theresponding node's performance. In some cases, the node 111 receives thetransmission metrics based on information included in the responses inregards to steps 704 a-704 c.

The node 111 may receive the responses in regards to steps 704 a-704 c,including the respective rankings and transmission metrics for the nodes113, 112, and 114. At step 706, the node 111 may determine a ranking foritself based on the transmission metrics. For example, the node 111 maydetermine for itself the example ranking of 20 indicated in Table 2. Theranking determined at step 706 may indicate the ability of the node 111to transmit messages through the RPL network 100 relative to additionalnodes in the RPL network 100, including one or more of the nodes 112,113, or 114.

At step 708, the node 111 may receive from the node 113 aneighbor-discovery message. For example, the neighbor-discovery messagereceived at step 708 may be a multicast neighbor-discovery messageindicating that the node 113 is seeking a parent node. In addition, theneighbor-discovery message received at step 708 may include a ranking ofthe node 113, such as the example ranking of 10 indicated for the node113 in Table 2.

At step 710, the node 111 may compare its own ranking to the rankingreceived in regards to step 708. For example, the node 111 may comparethe numeric value of 20 to the numeric value of 10. In addition, thenode 111 may determine that the node 113 has a higher reliability ascompared to node 111, based on the respective rankings of the nodes 113and 111 (e.g., a ranking of 10 indicates a higher reliability ascompared to a ranking of 20).

At step 712, the node 111 may transmit to the node 113 a response withan error code, such as an error code indicating that the node 111 is notavailable as a parent for the node 113. The response with the error codemay be transmitted at step 712 based on the comparison of the rankingsperformed at step 710. In some cases, transmitting an error code inresponse to a neighbor-discovery message received from a node having ahigher-ranked reliability may prevent a loop from forming in the RPLnetwork 100, such as a loop between the relatively high-reliability node113 and the relatively low-reliability node 111.

At step 730, the node 111 may receive a neighbor-discovery message fromthe node 112. The neighbor-discovery message may include a ranking ofthe node 112, such as the example ranking of 34 indicated for the node112 in Table 2.

At step 732, the node 111 may compare its own ranking to the rankingreceived in regards to step 730. For example, the node 111 may comparethe numeric value of 20 to the numeric value of 34. In addition, thenode 111 may determine that the node 112 has a lower reliability ascompared to node 111, based on respective rankings of the nodes 112 and111 (e.g., a ranking of 34 indicates a lower reliability as compared toa ranking of 20).

At step 734, the node 111 may transmit to the node 112 a response to theneighbor-discovery message received at step 730. The responsetransmitted at step 734 may indicate an availability of the node 111 asa parent node for the node 112. The response indicating the availabilitymay be transmitted based on the comparison of the rankings performed atstep 732.

In some cases, a neighbor relationship is established based on acomparison of node rankings. At step 736 for example, the node 112 mayestablish a neighbor relationship with respect to the node 111,responsive to the indicated availability at step 734. The neighborrelationship established at step 736 may indicate a parent status ofnode 111 and a child status of node 112.

In some cases, transmitting an error code in response to aneighbor-discovery message received from a node having a higher-rankedreliability may prevent a loop from forming in the RPL network 100, suchas a loop between a node with a relatively higher reliability and a nodewith a relatively lower reliability. In addition, transmitting the errorcode in a response to the higher-reliability node may prevent a loopfrom forming between more than two nodes, such as between a parent node,a child of the parent node, and a child of the child node (e.g., a loopbetween a “grandparent” node and a “grandchild” node). For example, thenode 111 may transmit a neighbor-discovery message to node 115 that is achild of node 112, as described in regards to FIG. 1b . Node 115 doesnot have a parent-child relationship with node 111 (according to theexample depicted in FIG. 1b ). If the node 111 were to establish aparent relationship with node 115, a three-node loop may be formed,where node 111 would be the parent of node 112, node 112 would be theparent of node 115, and node 115 would be the parent of node 111. In theRPL network 100, however, the node 115 determines that node 111 has arelatively higher reliability based on a comparison of the rankings ofnodes 111 and 115. The node 115 transmits a response that includes anerror code, and the node 111 establishes a parent relationship with anadditional node, such as with node 114. Therefore, responding to aneighbor-discovery message based on a comparison of node rankings mayprevent formation of loops in the RPL network 100.

In some implementations, a node in an RPL network may adjust its ownranking, such as an adjustment based on whether the node has a parent inthe RPL network. In some cases, an adjustment to a ranking includes aprocess for determining an infinite ranking (e.g., infiniteunreliability), such as an RPL poisoning process described by thestandard RFC 6550 for “RPL: IPv6 Routing Protocol for Low-Power andLossy Networks.” For example, if the RPL network 100 implements RPLpoisoning, the node 113 may adjust its ranking to reflect a very poorreliability, such as a ranking that is interpreted by other nodes asinfinite. Subsequent to receiving a response with an error code, thenode 113 may establish a parent relationship with another node, such asnode 111, based on the adjusted ranking. For example, an additionalcomparison of the ranking of the node 111 with the adjusted ranking ofthe node 113 (e.g., a ranking of 20 compared to an infinite ranking)indicates that the adjusted node 113 has a lesser reliability ascompared to the node 111. In addition, the node 111 may provide aresponse indicating that it is available as a parent to the node 113,based on the additional comparison.

FIG. 8 is a flow chart depicting an example of a process 800 forpreventing loop formation based on a comparison of node rankings. Insome implementations, such as described in regards to FIGS. 1-7, acomputing device configured as a node in an RPL network implementsoperations described in FIG. 8, by executing suitable program code. Forillustrative purposes, the process 800 is described with reference tothe examples depicted in FIGS. 1-7. Other implementations, however, arepossible.

At block 810, the process 800 involves transmitting a neighbor-discoverymessage in an RPL network. The neighbor-discovery message may betransmitted by a first node in the RPL network, such as the node 111 inthe RPL network 100. In addition, the neighbor-discovery message may betransmitted to additional nodes in the RPL network, such as to one ormore of nodes 112-119. The neighbor-discovery message may be transmittedas a multicast message, or indicated as not available forretransmission.

At block 820, the process 800 involves receiving, by the first node,responses from one or more additional nodes in the RPL network. Eachresponse may indicate a ranking of the respective responding node. Inaddition, each response may indicate transmission metrics of therespective responding node, such as transmission metrics describing areliability of a communication path for each respective node in the RPLnetwork. For example, node 111 may receive responses from one or more ofnodes 112, 113, or 114, indicating respective transmission metrics. Thetransmission metrics may indicate a reliability with which the nodes112, 113, or 114 transmit or route messages to the root node 110.

At block 830, the process 800 involves determining, by the node, a firstranking of the first node. The first ranking may indicate a reliabilityof the first node's communication path, such as a communication pathbetween the first node and a root node of the RPL network. For example,the node 111 may determine a ranking for itself that indicates areliability with which node 111 transmits or routes messages to the rootnode 110. In some implementations, the first ranking may be determinedbased, at least in part, on the transmission metrics received in regardsto block 820. For example, the node 111 may determine a ranking foritself based, in part, on transmission metrics of nodes that may routemessages from node 111 to the root node 110.

At block 840, the process 800 involves receiving, by the first node, asecond neighbor-discovery message from a second node in the RPL network.The second neighbor-discovery message may indicate a second ranking ofthe second node. For example, the node 111 may receive from node 113 aneighbor-discovery message indicating a ranking of node 113, such as inregards to step 708.

At block 850, the process 800 involves determining, by the first node,that the second node has a higher reliability than the first node. Thedetermination may be based on the first ranking and second ranking, suchas based on a comparison of values indicated by the rankings. Forexample, the node 111 may compare its own ranking to the ranking of node113 in regards to step 710, such as by performing a numeric comparisonof the values of the rankings. Based on the comparison, the node 111 maydetermine that the node 113 has a higher reliability.

At block 860, the process 800 involves transmitting, by the first nodeand to the second node, a response to the second neighbor-discoverymessage. The response may include an error code, such as an error codeindicating that the first node is not available as a parent for thesecond node. In regards to step 712 for instance, node 111 may transmitto node 113 a response with an error code that indicates node 111 is notavailable as a parent for node 113.

At block 870, the process 800 involves receiving, by the first node, athird neighbor-discovery message from a third node in the RPL network.The third neighbor-discovery message may indicate a third ranking of thethird node. For example, the node 111 may receive from node 112 aneighbor-discovery message indicating a ranking of node 112, such as inregards to step 730.

At block 880, the process 800 involves determining, by the first node,that the third node has a lower reliability than the first node. Thedetermination may be based on the first ranking and third ranking, suchas based on a comparison of values indicated by the rankings. Forexample, the node 111 may compare its own ranking to the ranking of node112 in regards to step 732, such as by performing a numeric comparisonof the ranking values. Based on the comparison, the node 111 maydetermine that the node 112 has a lower reliability.

At block 890, the process 800 involves transmitting, by the first nodeand to the third node, a response to the third neighbor-discoverymessage. The response may indicate that the first node is available as aparent for the third node. In some cases, the third node may establish aparent-child relationship based on the response to the thirdneighbor-discovery message. For example, responsive to a responseindicating the availability of node 111, transmitted in regards to step734, the node 112 may establish a neighbor relationship indicating aparent status of node 111 and a child status of node 112, such as atstep 736.

Although the blocks 870, 880, and 890 are described in regards to blocks810 through 860, other implementations are possible. For example, a nodethat transmits, based on a ranking comparison, a response indicating itsavailability as a parent, such as generally described in regards toblocks 870 through 890, may, but need not, perform another rankingcomparison or transmit another response with an error code, such asgenerally described in regards to blocks 810 through 860.

In some cases, one or more operations described in relation to any ofFIG. 3, 4, 6, or 8 may be performed in addition to, or in combinationwith, additional operations described in relation to another processdescribe in FIG. 3, 4, 6, or 8. For example, a node that withholdsneighbor-detection messages in an RPL network (as generally described inregards to FIG. 3) may also modify a neighbor relationship with anadditional node (as generally described in regards to FIG. 4). Inaddition, a node that transmits a response to a neighbor-discoverymessage based on a rankings comparison (as generally described inregards to FIG. 8) may also transmit a response to a neighbor-discoverymessage based on a parent-child status (as generally described inregards to FIG. 6). Additional implementations including additionalcombinations of the described operations are possible.

Example Node Configuration

Any suitable computing system or group of computing systems can be usedfor performing the operations described herein. For example, FIG. 9 is ablock diagram depicting a computing device 901 that is configured tooperate as a node in an RPL network, such as the RPL network 100,according to certain implementations.

The depicted example of the computing device 901 includes one or moreprocessors 902 communicatively coupled to one or more memory devices904. The processor 902 executes computer-executable program code oraccesses information stored in the memory device 904. Examples ofprocessor 902 include a microprocessor, an application-specificintegrated circuit (“ASIC”), a field-programmable gate array (“FPGA”),or other suitable processing device. The processor 902 can include anynumber of processing devices, including one.

The memory device 904 includes any suitable non-transitorycomputer-readable medium for storing a network table 930, aneighbor-discovery message 940, a response 950, and other received ordetermined values or data objects. The network table 930 may indicate,for example, information about additional nodes in the RPL network 100,such as rankings, response times, routes, neighbor relationships,parent-child status, or other suitable information. Thecomputer-readable medium can include any electronic, optical, magnetic,or other storage device capable of providing a processor withcomputer-readable instructions or other program code. Non-limitingexamples of a computer-readable medium include a magnetic disk, a memorychip, a ROM, a RAM, an ASIC, optical storage, magnetic tape or othermagnetic storage, or any other medium from which a processing device canread instructions. The instructions may include processor-specificinstructions generated by a compiler or an interpreter from code writtenin any suitable computer-programming language, including, for example,C, C++, C#, Visual Basic, Java, Python, Perl, JavaScript, andActionScript.

The computing device 901 may also include a number of external orinternal devices such as input or output devices. For example, thecomputing device 901 is shown with an input/output (“I/O”) interface 908that can receive input from input devices or provide output to outputdevices. A bus 906 can also be included in the computing device 901. Thebus 906 can communicatively couple one or more components of thecomputing device 901.

The computing device 901 executes program code that configures theprocessor 902 to perform one or more of the operations described abovewith respect to FIGS. 1-8. The program code includes operations relatedto, for example, one or more of the network table 930, theneighbor-discovery message 940, the response 950, or other suitableapplications or memory structures that perform one or more operationsdescribed herein. The program code may be resident in the memory device904 or any suitable computer-readable medium and may be executed by theprocessor 902 or any other suitable processor. In some implementations,the program code described above, the network table 930, theneighbor-discovery message 940, and the response 950 are stored in thememory device 904, as depicted in FIG. 9.

The computing device 901 depicted in FIG. 9 also includes at least onenetwork interface 910. The network interface 910 includes any device orgroup of devices suitable for establishing a wired or wireless dataconnection to one or more of the additional nodes in the RPL network100. Non-limiting examples of the network interface 910 include a Wi-Fimodem, a Bluetooth modem, a mobile data modem, an Ethernet networkadapter, and/or the like. The computing device 901 is able tocommunicate with the one or more additional nodes in the RPL network 100using the network interface 910.

GENERAL CONSIDERATIONS

Numerous specific details are set forth herein to provide a thoroughunderstanding of the claimed subject matter. However, those skilled inthe art will understand that the claimed subject matter may be practicedwithout these specific details. In other instances, methods,apparatuses, or systems that would be known by one of ordinary skillhave not been described in detail so as not to obscure claimed subjectmatter.

Unless specifically stated otherwise, it is appreciated that throughoutthis specification discussions utilizing terms such as “processing,”“computing,” “calculating,” “determining,” and “identifying” or the likerefer to actions or processes of a computing device, such as one or morecomputers or a similar electronic computing device or devices, thatmanipulate or transform data represented as physical electronic ormagnetic quantities within memories, registers, or other informationstorage devices, transmission devices, or display devices of thecomputing platform.

The system or systems discussed herein are not limited to any particularhardware architecture or configuration. A computing device can includeany suitable arrangement of components that provides a resultconditioned on one or more inputs. Suitable computing devices includemultipurpose microprocessor-based computer systems accessing storedsoftware that programs or configures the computing system from a generalpurpose computing apparatus to a specialized computing apparatusimplementing one or more implementations of the present subject matter.Any suitable programming, scripting, or other type of language orcombinations of languages may be used to implement the teachingscontained herein in software to be used in programming or configuring acomputing device.

Implementations of the methods disclosed herein may be performed in theoperation of such computing devices. The order of the blocks presentedin the examples above can be varied for example, blocks can bere-ordered, combined, and/or broken into sub-blocks. Certain blocks orprocesses can be performed in parallel.

The use of “adapted to” or “configured to” herein is meant as open andinclusive language that does not foreclose devices adapted to orconfigured to perform additional tasks or steps. Additionally, the useof “based on” is meant to be open and inclusive, in that a process,step, calculation, or other action “based on” one or more recitedconditions or values may, in practice, be based on additional conditionsor values beyond those recited. Headings, lists, and numbering includedherein are for ease of explanation only and are not meant to belimiting.

While the present subject matter has been described in detail withrespect to specific implementations thereof, it will be appreciated thatthose skilled in the art, upon attaining an understanding of theforegoing, may readily produce alterations to, variations of, andequivalents to such implementations. Accordingly, it should beunderstood that the present disclosure has been presented for purposesof example rather than limitation, and does not preclude inclusion ofsuch modifications, variations, and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the art.

What is claimed is:
 1. A method of using an IPv6 routing protocol forlow-power and lossy networks (“RPL”) to manage neighbor-detectionmessages in a computer network using an RPL protocol (“RPL network”),the method including operations executed by a processor of a nodeincluded in the RPL network, the operations comprising: determining, bythe node, a group of neighbor nodes included in the RPL network, whereineach of the neighbor nodes is configured to receive a neighbor-detectionmessage from the node, and wherein the node is configured to manage aneighbor relationship with a respective one of the neighbor nodes viathe neighbor-detection message; establishing, by the node, a firstneighbor relationship with respect to a first neighbor node of the groupof neighbor nodes, wherein the first neighbor relationship indicates aparent status of the node and a child status of the first neighbor node;establishing, by the node, a second neighbor relationship with respectto a second neighbor node of the group of neighbor nodes, wherein thesecond neighbor relationship indicates a child status of the node and aparent status of the second neighbor node; transmitting, by the node,the neighbor-detection message to a subset of the group of neighbornodes, wherein the subset excludes the first neighbor node based on thechild status of the first neighbor node, and wherein the subset includesthe second neighbor node based on the parent status of the secondneighbor node; and responding, by the node, to an additionalneighbor-detection message received from the first neighbor node.
 2. Themethod of claim 1, the operations further comprising: receiving, fromthe second neighbor node, a neighbor-discovery message; determining,based on the second neighbor relationship with respect to the secondneighbor node, that the second neighbor node has the parent status withrespect to the node; and responsive to determining that the secondneighbor node has the parent status: modifying the second neighborrelationship, such that the modified second neighbor relationship (i)revokes the parent status of the second neighbor node, and (ii)indicates an additional parent status of the node and a child status ofthe second neighbor node, establishing a third neighbor relationshipwith respect to a third neighbor node of the group of neighbor nodes,wherein the third neighbor relationship indicates a child status of thenode and a parent status of the third neighbor node.
 3. The method ofclaim 1, the operations further comprising: generating, by the node, afirst table entry describing the first neighbor relationship and asecond table entry describing the second neighbor relationship; andstoring the first table entry and the second table entry in a lookuptable of the node.
 4. The method of claim 1, the operations furthercomprising: modifying, by the node, the first neighbor relationship,such that the modified first neighbor relationship revokes the childstatus of the first neighbor node; transmitting, by the node, anadditional neighbor-detection message to an additional subset of thegroup of neighbor nodes, wherein the additional subset includes thefirst neighbor node based on the child status of the first neighbor nodehaving been revoked.
 5. The method of claim 1, the operations furthercomprising: transmitting, by the node, a neighbor-discovery message tomultiple additional nodes included in the RPL network; and receiving,from at least the first neighbor node and the second neighbor node,responses to the neighbor-discovery message, wherein the first neighborrelationship and the second neighbor relationship are established basedon the responses to the neighbor-discovery message.
 6. The method ofclaim 5, the operations further comprising: determining, based on theresponses to the neighbor-discovery message, transmission metrics of thesecond neighbor node and additional transmission metrics of the firstneighbor node; and based on the parent status of the second neighbornode, storing in a lookup table of the node the transmission metrics ofthe second neighbor node.
 7. A system for establishing neighborrelationships in a computer network using an IPv6 routing protocol forlow-power and lossy networks (“RPL network”), the system including: afirst node included in the RPL network, the first node having a firstneighbor relationship with respect to a second node included in the RPLnetwork and a second neighbor relationship with respect to a third nodeincluded in the RPL network, wherein the first neighbor relationshipindicates a parent status of the first node and a child status of thesecond node and the second neighbor relationship indicates a childstatus of the first node and a parent status of the third node, whereinthe first node includes a processor that is configured to performoperations comprising: receiving, from the third node, a firstneighbor-discovery message, wherein the first neighbor-discovery messageindicates a requested modification to the second neighbor relationship;determining, based on the second neighbor relationship with respect tothe third node, that the third node has the parent status with respectto the first node; and responsive to determining that the third node hasthe parent status, transmitting to the third node a first response tothe first neighbor-discovery message, the first response including anerror code, wherein the error code indicates that the requestedmodification is not available.
 8. The system of claim 7, the processorfurther configured for: responsive to determining a lost connection withthe third node, transmitting a second neighbor-discovery message to thesecond node; receiving, from the second node, a second response to thesecond neighbor-discovery message, the second response including theerror code; and responsive to the second response including the errorcode, establishing a third neighbor relationship with respect to afourth node included in the RPL network, wherein the third neighborrelationship indicates a child status of the first node and a parentstatus of the fourth node.
 9. The system of claim 8, the processorfurther configured for: determining, responsive to receiving the errorcode from the second node, a lifetime registration counter associatedwith the second node; during a time duration of the lifetimeregistration counter, transmitting no additional neighbor-discoverymessages to the second node.
 10. The system of claim 8, the processorfurther configured for: transmitting, responsive to receiving the errorcode from the second node, a deregistration message to the second node,wherein the deregistration message indicates a revocation of the childstatus of the second node; subsequent to sending the deregistrationmessage, transmitting an additional neighbor-discovery message to thesecond node.
 11. The system of claim 8, wherein determining the lostconnection includes: transmitting a third neighbor-discovery message tothe third node; and determining an expiration of a timeout thresholdprior to receiving a third response to the third neighbor-detectionmessage.
 12. The system of claim 7, wherein the error code is a codeindicating a status of a memory cache.
 13. The system of claim 7,wherein the error code is a code indicating an absence of a networkroute.
 14. A system for establishing neighbor relationships in acomputer network using an IPv6 routing protocol for low-power and lossynetworks (“RPL network”), the system including: a first node included inthe RPL network, wherein the first node includes a processor that isconfigured to perform operations comprising: transmitting, by the firstnode, a first neighbor-discovery message to multiple additional nodesincluded in the RPL network; receiving, from each node of the multiplenodes, a respective response indicating (i) a ranking indicating arespective reliability of the respective node and (ii) transmissionmetrics for the respective node; determining, based on the transmissionmetrics, a first ranking of the first node, the first ranking indicatinga reliability of a communication path between the first node and a rootnode of the RPL network, wherein the first ranking of the first node andthe ranking of each respective node indicates a relative reliability ofthe first node as compared to each respective node; receiving from asecond node of the multiple nodes, a second neighbor-discovery messageindicating a second ranking of the second node; determining, based onthe first ranking and the second ranking, that the second node has ahigher reliability than the first node; responsive to determining thatthe second node has the higher reliability, transmitting to the secondnode a response to the second neighbor-discovery message, the responseincluding an error code.
 15. The system of claim 14, the processorfurther configured for: receiving, from a third node of the multiplenodes, a third neighbor-discovery message indicating a third ranking ofthe third node; determining, based on the first ranking and the thirdranking, that the third node has a lower reliability than the firstnode; and responsive to determining that the third node has the lesserreliability, transmitting to the third node an additional response tothe third neighbor-discovery message, the additional response indicatingan availability of the first node as a parent node.
 16. The system ofclaim 15, wherein the third node is configured to establish, responsiveto the additional response indicating the availability of the firstnode, a neighbor relationship with respect to the first node indicatinga parent status of the first node and a child status of the third node.17. The system of claim 14, wherein determining that the second node hasa higher reliability than the first node includes comparing a numericvalue of the first ranking to a numeric value of the second ranking. 18.The system of claim 14, wherein the RPL network implements RPLpoisoning.
 19. The system of claim 14, wherein the error code is a codeindicating either a status of a memory cache or an absence of a networkroute.
 20. The system of claim 14, wherein the first neighbor-discoverymessage is transmitted as a multicast message and the response to thesecond neighbor-discovery message is transmitted as a unicast message.